1. Field of the Invention
The present invention relates to a fault localization apparatus for an optical line, and more particularly to a fault localization apparatus for an optical line in a wavelength division multiplexed passive optical network (WDM PON) in which a downstream light source of a channel having a fault, as a monitoring light source, is used.
2. Description of the Related Art
Recently, as various data services are rapidly increased through the Internet, a large amount of transmission capacity is needed in a subscriber network. In order to efficiently comply with such requirement, a wavelength division multiplexed passive optical network (WDM PON) has attracted considerable attention. Here, the WDM PON is capable of providing a large number of services as optical signals whose wavelengths are different from each other are provided to each subscriber, and a plurality of optical signals are performed by wavelength division multiplexing to be transmitted over a single optical line. Such a WDM PON has advantages in that, as networks are implemented with passive elements to comply with fast transmission of information, networks can be easily installed, maintained and administered, and also extension and security are high. Furthermore, the WDM PON can provide various kinds of services according to wavelengths.
Although such a WDM PON has the above advantages, when substantially implementing the WDM PON, fist of all, economical efficiency and reliability therefor must be considered. In order to efficiently configure the network, various light sources for a WDM PON and methods for cost-effectively implementing elements employed in the network have been researched. Although a large amount of data can be transmitted through the WDM PON, compared with the prior art networks, when a fault occurs in the WDM PON, effects thereof are very serious. Therefore, securing network reliability is very important. In order to improve network reliability, it is necessary to immediately detect any fault in the network and rapidly restore such a detected fault. Also, when improving such network reliability, operations, such as fault detection, sensing of fault position and fault restoration, should be cost-effectively performed.
FIG. 1 is a block diagram of a wavelength division multiplexed passive optical network according to the prior art.
The prior art WDM PON includes a central office CO, a remote node RN and a plurality of optical network units. The central office includes transmitters, receivers and a multiplexer and a demultiplexer having arrayed waveguide gratings (hereinafter, AWGs), such that downstream optical signals which have different wavelengths and are transmitted from the transmitters can be multiplexed by the multiplexers, and then transmitted to the remote nodes through optical lines. The multiplexers/demultiplexers installed in the remote nodes demultiplex the multiplexed downstream optical signals and transmit the optical signals with different wavelengths to each optical network unit. Downstream receivers installed in each optical network unit transform the downstream optical signals to electrical signals to perform communication using the same.
Conversely, upstream optical signals transmitted from the transmitters installed in optical network units are multiplexed by the multiplexer/demultiplexer of the remote nodes, and then transmitted in the upstream direction. After passing through a WDM coupler classifying upstream and downstream optical signals in the central office, the optical signals are demultiplexed by the demultiplexer in the central office, and then are received by the upstream receivers.
A fault in an optical line is detected by determination as to whether the upstream optical signals are normally received. In order to detect such a fault, a monitor is additionally needed therein.
The position of a fault in optical lines connecting the central office with the remote nodes can be detected using a monitor having a single wavelength, since optical signals whose their wavelengths are multiplexed are passes through the same optical line. However, since the optical lines connecting the remote nodes with the optical network units pass optical signals whose wavelengths are different from each other therethrough, a monitor, which has a plurality of light sources to comply with the wavelengths of the optical lines or a light source whose wavelength can be varied, is used in order to detect positions of optical line faults. [Reference: Kuniaki Tanaka, et al., “In-service Individual Line Monitoring and a Method for Compensating for the Temperature-dependent Channel-Drift of a WDM-PON Containing an AWGR Using a 1.6 μm Tunable OTDR,” European Conference on Optical Communication (ECOC '97), no. 488, pp. 295-298, September 1997.] Otherwise, the apparatus must be constructed such that path of signal light and monitoring light is separated in front of the multiplexer/demultiplexer of the remote node, and then the separated monitoring light is recoupled in a path used by a channel of each optical network unit after the multiplexer/demultimplexer.
However, the apparatus having such a structure has disadvantages in that a relatively expensive light source whose wavelength is tunable must be used or network complexity is increased since a plurality of light sources must be used, thereby decreasing network cost-effectiveness.